Selective fluorocarbon-based RIE process utilizing a nitrogen additive

ABSTRACT

A CHF 3  -based RIE etching process is disclosed using a nitrogen additive to provide high selectivity of SiO 2  or PSG to Al 2  O 3 , low chamfering of a photoresist mask, and low RIE lag. The process uses a pressure in the range of about 200-1,000 mTorr, and an appropriate RF bias power, selected based on the size of the substrate being etched. The substrate mounting pedestal is preferably maintained at a temperature of about 0° C. Nitrogen can be provided from a nitrogen-containing molecule, or as N 2 . He gas can be added to the gas mixture to enhance the RIE lag-reducing effect of the nitrogen.

FIELD OF THE INVENTION

The present invention relates generally to etching and more particularlyto the selective reactive ion etching of silicon dioxide andphosphosilicate glass (PSG).

BACKGROUND OF THE INVENTION

Known plasma or reactive ion etching (RIE) processes for selectivelyetching silicon dioxide (or PSG) layers on semiconductor chips requireheretofor inadequate trade-offs between chamfering of the photoresist,etch rate selectivity between the SiO₂ layer and an etch stop, and "RIElag" (the phenomenon where large area via holes are etched more rapidlythan small area via holes on the same chip). Such RIE processes aretypically optimized for a particular use by varying the chemistry,pressure, flow rate, and power parameters of the environment.

High pressure (i.e., pressures greater than 1 Torr) fluorocarbon RIEprocesses are desirable in that they provide good selectivity of silicondioxide. Such processes include, for example, CHF₃ +CF₄ +Ar, and CHF₃+He chemistries. However, these high pressure processes have been foundto provide particularly poor RIE lag characteristics.

Low pressure fluorocarbon processes are desirable for selective etchingof silicon dioxide, in that they provide less RIE lag than the highpressure processes described above. Such processes include, for exampleCHF₃ +He chemistry at 0.5-100 mTorr pressure. In fact, such processesfunction adequately in low density, low power, batch reactors. Suchprocesses, however, provide unacceptable chamfering of photoresist masklayers when used in higher power, single wafer reactors. This chamferingresults in loss of critical dimensions, requiring the use of undesirablylarge ground rules.

Low pressure, high density processes, which utilize plasma generationmechanisms such as magnetrons, inductive coupling, or electron cyclotronresonance, reduce the photoresist mask chamfering typically found withthe lower pressure processes. However, this class of RIE processesprovides poor selectivity to the silicon dioxide, and may result inpolymer-induced RIE lag within small via openings.

Anisotropic silicon etching (whereby near-vertical etched siliconprofiles are achieved) using NF₃ or SF₆ as a primary etchant, and CHF₃and N₂ as additives, are known in the art. See U.S. Pat. No. 4,741,799,issued on May 3, 1988, to Lee Chen et al. and assigned to the presentassignee. This particular etching system is useful for selectivelyetching silicon through a silicon dioxide mask.

In U.S. Pat. No. 4,767,724, issued on Aug. 30, 1988, to Manjin J. Kim etal., a mixture of CHF₃ and argon is mentioned as "not the preferredetching gas" for etching vias in silicon dioxide down to an aluminumoxide etch-stop layer. For high selectivity between the etching rates ofSiO₂ and Al₂ O₃, Kim et al. recommends a gas mixture of NF₃ and argon.

Nitrogen trifluoride with argon, but without a carbon-containing gassuch as CHF₃, is shown in U.S. Pat. No. 4,904,341, issued on Feb. 27,1990, to Richard D. Blangher et al., for etching SiO₂ while avoidingunwanted polymer by-products which carboncontaining etch gases are proneto leave on the circuit workpiece and on the walls of the reactor.

Etching of oxide selectively to tantalum with mixtures of fluorinecontaining and nitrogen containing molecules is taught in Japanese KokaiNo. 1-33323.

While various high and low pressure, fluorocarbon-based, RIE processesare known in the art, all of these processes require an unacceptabletrade-off between selectivity, mask chamfering, and RIE lag. No processis known for selectively etching SiO₂ (or PSG) which simultaneouslyyields acceptable photoresist chamfering, high selectivity, and good(i.e. low) "RIE lag" characteristics.

OBJECTS OF THE INVENTION

An object of the present invention is to provide a new and improvedprocess and apparatus for selectively etching SiO₂ or PSG over an etchstop layer, this process and apparatus simultaneously providing lowphotoresist (i.e. mask) chamfering, high selectivity between the etchedlayer and an underlying etch stop layer, and low RIE lag.

Another object of the present invention is to provide such a process andapparatus with which vias of disparate dimensions (i.e. less than 1micron and greater than 30 microns) can be etched without degrading theintegrity of the etch stop layer.

A further object of the present invention is to provide such anapparatus and process which can etch vias of disparate depths (i.e. from0.2-4.0 microns) while preserving the integrity of the underlying etchstop layer.

Another object of the present invention is to provide a CHF₃ -based RIEprocess suitable for selectively etching SiO₂ or PSG over one of severalselected etch stops on integrated circuit chips.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, there is provided areactive ion etching environment for selectively etching SiO₂ or PSGover an etch stop material on a workpiece in an etching chamber,comprising: a gas mixture including CHF₃ and a nitrogen containingmolecule; and, the gas mixture maintained at a pressure in the range ofabout 200 to about 1000 mTorr. The nitrogen containing molecule ispreferably N₂. The etch stop material is preferably selected from thegroup consisting of magnesium oxide, aluminum oxide, silicon, andtitanium silicide. The etching environment further preferably includes apedestal for mounting the workpiece at a temperature below about 0° C.RF power is selected appropriately for the wafer size, for example inthe range of about 500-700 watts for a 125 mm substrate. Helium can beselectively added to the gas mixture to even further reduce the RIE lag.

In accordance with another aspect of the invention, there is provided areactive ion etching method for selectively etching SiO₂ or PSG over anetch stop material on a workpiece in an etching chamber, comprising thesteps of: introducing a gas mixture including CHF₃ and a nitrogencontaining molecule into the chamber; and, maintaining the gas mixtureat a pressure in the range of about 200 to about 1,000 mtorr. Thenitrogen containing molecule is preferably N₂. The etch stop material ispreferably selected from the group consisting of magnesium oxide,aluminum oxide, silicon, and titanium silicide. The etching processfurther preferably includes a pedestal for mounting the workpiece at atemperature below about 0° C., and a source of RF power in the range ofabout 500-700 watts for exciting the gas mixture. Helium can be added tothe gas mixture to even further reduce the RIE lag.

DETAILED DESCRIPTION OF THE INVENTION

The present inventors have discovered that high selectivity of etchedSiO₂ or PSG, to an etch stop such as Al₂ O₃, can be achieved byinhibiting the etching of the etch stop material, which in turn can beachieved by reducing the gas radical and ion vibrational temperature ofa RIE etching plasma. The addition of a nitrogen containing molecule(such a molecule defined herein as including the pure nitrogen moleculeN₂), to a CHF₃ RIE gas plasma has been discovered to be beneficial inthis effect, in that nitrogen has vibrational states close to those ofthe fluorocarbons, and thus functions as a plasma vibrational energysink. Moreover, it is theorized that, when such nitrogen is added to theCHF₃ gas plasma, a thin layer of AlN forms to further inhibit theetching of the Al₂ O₃ etch stop layer. The present inventors havefurther discovered that the addition of He to the gas mixture furtherenhances the desirable effect of nitrogen in the fluorocarbon etchinggas, and that these beneficial effects are achieved at a pressure in therange of 200-1,000 mTorr, a "mid-range" pressure in comparison to thebackground art discussed above.

The present invention is more specifically directed to a RIE process forselectively etching SiO₂ or PSG over an etch stop material, utilizing aCHF₃ /nitrogen-containing-molecule gas mixture and the followingparameters: a pressure in the range of about 200-1,000 mTorr; anappropriate RF bias power, for example in the range of about 500-700watts for a 125 mm substrate; and a single wafer pedestal temperature ofbelow about 0° C. For other wafer sizes, RF bias power is appropriatelyscaled to provide equivalent current and power density levels. He gascan be selectively added to the gas mixture, up to approximately 75% ofthe total gas mixture, to further enhance the beneficial effects of thenitrogen.

The etch stop material is selected from the group comprising aluminumoxide (Al₂ O₃), magnesium oxide (MgO), silicon and titanium silicide(TiSi). The latter two materials may have slightly lower etch rateratios than that of Al₂ O₃, but in general would exceed a desirableratio of 20:1.

The present inventors have discovered that, with the materials andprocess parameters set out above, the RIE process yields a desirablyhigh etched layer/etch stop, etch rate selectivity of greater than 20:1(i.e. greater than 50:1 for an Al₂ O₃ etch stop), a desirably low RIElag of below about 20%, and a desirably steep via profile in the etchedlayer of substantially 90°.

The results shown in Table A were obtained when the indicated plasmaetching parameter values were used in a single wafer etching chamber. Asuitable chamber is manufactured and sold by Applied MaterialsCorporation under Model No. 5000, and may be routinely modified by theuser to obtain the operating parameters specified in Table A. In allthirteen examples set forth in Table A the etched layer is SiO₂ the etchstop material utilized is Al₂ O₃ and the temperature of the pedestal formounting the wafer is 0° C. In example 1, a single layer resist etchrate of 600 Å/minute and a profile degree of 88°-90° were measured.Similar results can be expected for examples 2-13 but were not measured.In example 1, an RIE lag of 16% for 0.9 micrometer vs. 1.6 micrometervia diameters was measured. As is pointed out in the Background of theInvention, it is known that etch processes operating at lower pressuresprovide less RIE lag. Since examples 2-13 are at lower pressures thanexample 1, it is therefore assumed that any RIE lag in these exampleswould be equal to or less than 16%.

                                      TABLE A                                     __________________________________________________________________________                  EXAMPLE NO.:                                                                  1  2  3  4  5  6  7  8  9  10 11 12 13                          __________________________________________________________________________    PARAMETERS:                                                                   CHF.sub.3 Flow Rate (sccm)                                                                  40 40 40 40 10 40 60 10 40 60 40 40 40                          N.sub.2 Flow Rate (sccm)                                                                    60 60 60 60 90 60 40 90 60 40 60 60 60                          Gas Mixture Pressure (mT)                                                                   1000                                                                             900                                                                              900                                                                              900                                                                              900                                                                              900                                                                              900                                                                              500                                                                              500                                                                              500                                                                              700                                                                              500                                                                              500                         RF Bias Power (WATTS)                                                                       700                                                                              700                                                                              700                                                                              700                                                                              700                                                                              700                                                                              700                                                                              700                                                                              700                                                                              700                                                                              700                                                                              500                                                                              600                         Magnetic Field Strength                                                                        0  30 60 0  0  0  0  0  0  0  0  0                           (GAUSS)                                                                       RIE PROCESS RESULTS:                                                          Selectivity (SiO.sub.2 /A.sub.2 O.sub.3                                                     >50                                                                              84 93 84 28 94 240                                                                              22 44 49 64 146                                                                              59                          Etch Rate Ratio)                                                              SiO.sub.2 Etch Rate (Å/min)                                                             2700                                                                             2529                                                                             2524                                                                             2516                                                                             2125                                                                             2810                                                                             1920                                                                             2204                                                                             3770                                                                             3654                                                                             3212                                                                             1756                                                                             2787                        Al.sub.2 O.sub.3 Etch Rate (Å/min)                                                      45 30 27 30 76 30 8  101                                                                              85 75 50 12 47                          RIE Lag (%)   16                                                              Etch Rate Uniformity                                                                           4.5                                                                              4.5                                                                              7.8                                                                              3.48                                                                             4.6                                                                              14.3                                                                             3.7                                                                              3.19                                                                             3.5                                                                              3.19                                                                             6.3                                                                              2.8                         __________________________________________________________________________

Generally, for a given power and pressure, there is an optimum amount ofnitrogen that yields the above-described results. As power is increasedand pressure decreased, this optimum amount of nitrogen decreases as apercentage of the total gas mixture. As power is decreased and pressureis increased, this optimum amount of nitrogen increases as a percentageof the total gas mixture. Increased pressures generally provideincreased selectivity (with acceptable chamfering and RIE lag). However,if the pressure is increased outside of the range of the invention, theRIE lag becomes unacceptable. He can be added, up to approximately 75%of the total gas mixture, to further decrease the RIE lag, especially inthe higher pressure ranges.

The colder the temperature at which the wafer is maintained, the greaterthe selectivity that can be obtained with the present RIE process.Pedestal temperatures below 0° C., and increased wafer cooling, are thusadvantageous to the process. With an electrostatic chuck, He gas can beused to provide the thermal coupling between the chuck and the waferbeing etched.

Other nitrogen containing molecules such as ammonia can be used with orin place of N₂ to provide the nitrogen required by the RIE process asdescribed above.

The present inventors have thus provided a fluorocarbon-based RIEprocess for selectively etching silicon dioxide over an etch stop layer,the process simultaneously providing high etch rate selectivity, lowchamfering of the photoresist etching mask, and low RIE lag. The processhas application in the etching of SiO₂ or PSG on semiconductor chips,and particular application in such etching as used to fabricatevery-large scale integrated (VLSI) circuits.

While the present invention has been shown and described with respect toselected embodiments, it is not thus limited. Numerous modifications,changes, and improvements will occur to those skilled in the art andwhich are all within the spirit and scope of the invention.

What is claimed is:
 1. A method for selectively reactive ion etchingSiO₂ over an aluminum oxide etch stop material on a workpiece in anetching chamber, comprising the steps of:introducing a gas mixtureincluding CHF₃ and a nitrogen containing molecule into said chamber atflow rates of between 40 sccm and about 60 sccm; and maintaining saidgas mixture at a pressure in the range of 500 to 1,000 mtorr.
 2. Theetching process of claim 1 wherein said nitrogen containing molecule isN₂.
 3. The etching process of claim 1 and further including a pedestalfor mounting said workpiece, the temperature of said pedestal beingbelow 0° C.
 4. The etching process of claim 1 wherein:said workpiececonsists of a single wafer of 125 mm diameter; and further comprising asource of RF power in the range of about 500-700 watts for exciting saidgas-mixture.
 5. The etching process of claim 1 wherein said gas mixturefurther includes He.
 6. The etching process of claim 1 wherein the gasflow rates of said CHF₃ and N₂ are 40 and 60 sccm, respectively.